Linux Siemens R3964 Line Discipline Missing Lock

The Siemens R3964 line discipline code in drivers/tty/n_r3964.c has a few races around its ioctl handler; for example, the handler for R3964_ENABLE_SIGNALS just allocates and deletes elements in a linked list with zero locking. This code is reachable by an unprivileged user if the line discipline is enabled in the kernel config; Ubuntu 18.04, for example, ships this line discipline as a module.